1. Field of the Invention
The present invention relates to a frequency correction circuit and a frequency correction method in a receiver for receiving a radio signal provided in a radio base station and a mobile terminal used in a mobile communication system.
2. Description of the Related Art
In general, in a mobile communication system employing radio technology, communication between a radio base station and a mobile terminal is performed with a radio transmission channel.
As a multiple access system that the radio base station uses to simultaneously communicate with plural mobile terminals, Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), and the like are put to practical use.
In such a mobile communication system, it is likely that a frequency error is present in a received signal such as a shift of fixed radio frequencies between the radio base station and the mobile terminals. In this case, usually, an operation for correcting the frequency error by AFC (Auto Frequency Control) is performed in radio signal receivers of the radio base station and the mobile terminals. The operation for correcting the frequency error by AFC is carried out by a method of demodulating a received signal and detecting a frequency error to control a frequency of a local transmitter of a reception circuit or by a method of controlling a frequency offset added to a received signal. Loop control is mainly used for these methods.
Referring to FIG. 1, an example of a general frequency correction circuit including reception antenna unit 901, radio receiver 902, frequency-offset giving units 903-1 to 903-n, demodulators 904-1 to 904-n, demodulation-signal selector 906, and frequency-offset determining unit 907 is shown.
Reception antenna unit 901 receives a radio signal transmitted between a radio base station and a mobile terminal.
Radio receiver 902 is inputted with a signal outputted from reception antenna unit 901, performs amplification, frequency conversion from a radio band to a base band, orthogonal detection, analog to digital conversion, and the like for the inputted signal, and outputs the signal subjected to these kinds of processing to frequency-offset giving units 903-1 to 903-n. In general, radio receiver 902 includes a low-noise amplifier, a band limiting filter, a mixer, a local transmitter, an AGC (Auto Gain Controller), an orthogonal detector, a low-pass filter, and an analog to digital converter.
Frequency-offset giving units 903-1 to 903-n that are inputted with the signal outputted from radio receiver 902 and a signal outputted from frequency-offset determining unit 907, give frequency offsets outputted from frequency-offset determining unit 907 to the signal outputted from radio receiver 902, and output signals to demodulators 904-1 to 904-n, respectively. Specifically, frequency-offset giving units 903-1 to 903-n apply phase rotation processing that corresponds to the frequency offset given to the signal outputted from frequency-offset determining unit 907 to respective symbols of a digital received signal in the base band, which is the signal outputted from radio receiver 902.
Demodulators 904-1 to 904-n are inputted with the signals outputted from frequency-offset giving units 903-1 to 903-n, respectively, perform separation of desired user signal components from multiplexed plural user signal components, detection and selection of timing of multi-path components of the desired user signal components, i.e., path delays, channel estimation, and the like, and calculate demodulation signals. The demodulation signals are outputted to demodulation-signal selector 906.
Demodulation-signal selector 906 selects an optimum demodulation signal out of the demodulation signals outputted from demodulators 904-1 to 904-n. 
Frequency-offset determining unit 907 determines fixed frequency offsets corresponding to plural frequency errors that cover bands in which a frequency error is likely to occur and outputs the determined frequency offset to frequency-offset giving units 903-1 to 903-n. 
On the other hand, concerning a communication scheme in which the operation for correcting a frequency error by AFC employing loop control is difficult such as burst communication, a method of giving fixed frequency offsets corresponding to plural frequency errors prepared in advance to a received signal and, then, performing demodulation, and selecting an optimum signal out of the demodulation signals is disclosed in JP1995-176994A.
However, in the related art described above, in order to accurately correct a frequency error of a received signal and prevent deterioration at the time of demodulation, intervals of fixed frequency offsets corresponding to frequency errors have to be set sufficiently small to cover bands in which it is likely that a frequency error occurs.
When the intervals of the fixed frequency offsets corresponding to the frequency errors are set sufficiently small to over bands in which it is likely that an error occurs, it is necessary to prepare frequency-offset giving units and demodulators in a number of the fixed frequency offsets corresponding to the frequency errors. Therefore, the circuit size increases.